Generally, an inverter is a device that electrically converts a direct current (DC) to an alternate current (AC). An inverter used in the industry is defined as a series of devices that control a motor speed such that the motor be used with a high efficiency by varying a voltage and frequency of power supplied from a commercial power supply and injecting the varied power to the motor. Since the inverter may control a magnitude and frequency of AC voltage, the inverter has been widely used in systems requiring variable speed operations.
In a motor drive system to which the inverter is applied, when an input power to the inverter decreases due to an accident or the like, a magnitude of an inverter output current varies depending on a voltage synthesis performance of the inverter. Under a assumption of the same load, a magnitude of an output current decreases as a magnitude of an output voltage increases. Thus, the system loss may be reduced and a current fault may be prevented. As a result, studies have been actively made on an overmodulation scheme that can increase the output voltage of the inverter relative to a reduced input voltage.
The inverter overmodulation scheme may be divided into a dynamic overmodulation scheme and a static overmodulation scheme. The dynamic overmodulation scheme is mainly applied to vector control for instantaneous control. The static overmodulation scheme such as a V/f control scheme of an induction motor is mainly applied to a steady state operation.
For a conventional static overmodulation, when the output voltage of the inverter is increased, linearity cannot be secured in a region where a modulation index is equal to or larger than a predetermined value. The linearity means a property of synthesizing the same output voltage as a command voltage. When the linearity is not guaranteed, there is a problem that an output voltage smaller than the command voltage occurs in an overmodulation region. As a result, a driving of the load is limited.